Remove unused files from package

[f440]

This pull request updates the *.gemspec file to optimize the gem package size and structure. The changes include:

• Modified files to exclude test files, documentation files, Gemfile and CHANGELOG from the package.
• Add URLs for documentation and ChangeLog to the package metadata
  • Users can access this information using bundle info activerecord-multi-tenant

These changes reduce the extracted package size from 1.1 MB to 132 KB.

Benefits:

• Shorter download times for users
• Reduced container sizes when the gem is included

References:

• Don't package test files with gem rubygems/bundler#3207
• Template for the *.gemspec file generated by bundle gem command